Integration of optical circuit components has been established for a number of years. Two integration methodologies have been developed during this time. A first methodology involves the fabrication of dielectric waveguides on a silicon (Si) wafer substrate. A second methodology utilizes semiconductor material for fabricating waveguides in a GaAs/AlGaAs optical crystal.
Within the first methodology, several techniques have been reported for accomplishing fabrication of a dielectric waveguide on the silicon substrate. See, for example, W. Stutius et al., "Silicon nitide films on silicon for optical waveguides," Appl. Optics, Vol. 16, pp. 3218-3222, December 1977; G. Marx et al., "Integrated Optical Detector Array, Waveguide, and Modulator Based on Silicon Technology," IEEE J. of Solid-State Circuits, Vol. SC-12, pp. 10-13, February 1977; J. Boyd et al., "An Integrated Optical Waveguide and Charge-Coupled Device Image Array," IEEE J. of Quantum Electronics, Vol. QE-13, pp. 282-287, April 1977, and "Integrated optical silicon photodiode array," Appl. Optics, Vol. 15, pp. 1389-1393, June 1976. Stutius et al. show a silicon nitride (Si.sub.3 N.sub.4) thin film waveguide grown by low-pressure chemical vapor deposition on a silicon dioxide (SiO.sub.2) buffer layer. The SiO.sub.2 buffer layer is a steam oxide layer grown at 1100 degrees Centigrade in a conventional horizontal reactor. In the Marx et al. reference, a hybrid, i.e., nonmonolithic, integrated optical circuit is shown wherein a Corning 7059 glass waveguide film interconnects devices by taper coupling. The 7059 glass waveguide film is sputtered onto a SiO.sub.2 buffer layer which is thermally grown at high temperature on a silicon substrate. Boyd et al. describe an integrated optical component structure incorporating a taper-coupled, KPR photoresist waveguide spin-coated on a SiO.sub.2 buffer layer which is thermally grown at high temperature over a silicon substrate.
Although the techniques of Stutius et al., Marx et al., and Boyd et al. appear to offer approaches to integrating certain optical components with dielectric waveguides, their reliance on silicon technology and on high temperature thermal growth of the SiO.sub.2 buffer layer cause these techniques to be inapplicable for monolithic integration on optical crystals such as AlGaAs/GaAs and InGaAsP/InP heterostructures. Silicon technology is a limitation on the applicability of these techniques for monolithic integration because the bandgap structure of silicon is not conducive to fabrication of efficient active optical circuit components such as light sources on the silicon wafer substrate. Thermal growth is also a limitation on applicability because the temperatures involved in the thermal growth process are considerably higher than the melting point temperatures of optical crystals in the AlGaAs/GaAs system or the InGaAsP/InP system.
As mentioned above, the second integration methodology provides an approach for fabricating semiconductor waveguides in optical crystals of the AlGaAs/GaAs system. This methodology has resulted in the monolithic integration of active and passive optical circuit components such as light sources, modulators, amplifiers, detectors and couplers, as described in the references cited below. J. L. Merz et al., "Integrated GaAs-Al.sub.x Ga.sub.1-x As injection lasers and detectors with etched reflectors," Appl. Phys. Lett., Vol. 30, pp. 530-533, May 1977, disclose monolithic integration of a GaAs double heterostructure laser with a passive waveguide and an external cavity detector in a four layer GaAs-Al.sub.x Ga.sub.1-x As device. The integration of a detector or modulator with a large optical cavity, distributed Bragg reflector laser has been achieved by M. Shams et al. as disclosed in "Monolithic integration of GaAs-(GaAl)As light modulators and distributed-Bragg-reflector lasers," Appl. Phys. Lett., Vol. 32, pp. 314-316, March 1978. K. Aiki et al. in an article, "Frequency multiplexing light source with monolithically integrated distributed-feedback diode lasers," Appl. Phys. Lett., Vol. 29, pp. 506-508, October 1976, have fabricated six distributed feedback lasers on a single chip with the laser outputs, at different frequencies being multiplexed into a single waveguide. Utilizing an integrated twin guide structure, K. Kishino et al. have demonstrated the coupling of two devices to a passive waveguide in an article, "Monolithic integration of laser and amplifier/detector by twin-guide structure," Japan J. Appl. Phys., Vol. 17, pp. 589-590, March 1978.
For the approaches described above in relation to the second methodology, the passive waveguide is a layer of semiconductor material which is substantially transparent to the lightwaves conducted therein. Variations in the thickness and the refractive index of the waveguide layer as well as the coupling length of the device affect proper operation of the resulting integrated optical circuit. In order to control these variations, close monitoring is required for, and increases the complexity of, crystal growth processes employed in this integration methodology.
Although it is well known that dielectric waveguides of the first methodology are more efficient than semiconductor waveguides of the second methodology, the proponents of the two methodologies described above have failed to address the problem of fabricating a monolithically integrated optical circuit which includes dielectric optical waveguides.